Chaotic Overcasting

Chaotic clerics can overcast. Using a ritual which takes one turn and suffering Corruption (as per Shades of Magic), they may cast a spell one level above what they can usually cast. Risking greater corruption and requiring a "price" - typically human sacrifice - they can cast a spell two levels higher than what they usually can cast. This is available from level 1 and on.

The purpose here is to tempt the player in addition to his character. Once you get this power you start your downward spiral. Power is addictive.

Lawful clerics can do this as well. This will eventually cause alignment shift to Netral and then Chaotic. When you becone Neutral, you lose your Lawful clerical abilities. Chaos, of course, remains available. Once Corruption makes then Chaotic, they become Chaotic clerics. Going back is nearly impossible...